In recent years, researches and developments concerning optical information recording systems have been made actively in the field of information recording.
This optical information recording system has many advantages in which it can cope with various types of memories such as non-contact type recording and reproducing memories, read-only memories, write-once memories and rewritable memories, and hence a wide use from an industrial use to a consumer use of such system has been considered as a system that can realize inexpensive mass-storage files.
Increase of storage capacity of optical recording medium, for example, optical disc for use with various kinds of information recording systems has been realized mainly by reducing a wavelength of laser light serving as a light source for use in an optical recording medium and by adopting an objective lens with a high numerical aperture (N. A.) to decrease a size of a beam spot on a focus surface.
While a CD (compact disc) affords a storage capacity of 650 MB when it uses laser light with a wavelength of 780 nm and an objective lens having a numerical aperture (N. A.) of 0.45, a DVD-ROM (digital versatile disc read-only memory) affords a storage capacity of 4.7 GB when it uses laser light with a wavelength of 650 nm and an objective lens having a numerical aperture N. A. of 0.6.
Further, a next-generation optical system uses an optical disc with a thin light transmission layer having a thickness of about 0.1 mm formed on an optical recording layer and allows laser light to be irradiated thereon from the side of this light transmission layer so that a large storage capacity of greater than 22 GB can be realized by laser light having a wavelength less than 450 nm and an objective lens with a numerical aperture N. A. of 0.78 or more.
FIG. 6(a) is a schematic perspective view showing optical recording state or optical reproducing state in this optical disc.
An optical disc DC is shaped like a disk with a center hole CH bored at its center portion, and is rotated in the direction shown by an arrow DR, for example, in FIG. 6(a).
FIG. 6(b) is a schematic cross-sectional view of the optical disc. FIG. 6(C) is an enlarged cross-sectional view of a main portion of this optical disc DC.
This optical disc has a disc substrate 101 having a thickness of about 1.1 mm made of polycarbonate, for example, a concave portion 101r formed on one major surface of the disc substrate and an optical recording layer 102 formed along a concave and convex surface including the concave portion 101r. 
In a phase-change type optical disc DC, for example, its optical recording layer 102 is comprised of a laminated layer material consisting of a dielectric film, a phase-change film, a dielectric film and a reflective film and the like, for example.
A light transmission layer 103 having a thickness of 0.1 mm, for example, is formed on the optical recording layer 102.
When information is recorded on or reproduced from this optical disc DC, light LT of laser light having a wavelength not more than 450 nm, for example, in a range from 380 nm to 420 nm is focused and irradiated on the optical recording layer 102 from the side of the light transmission layer 103 of the optical disc DC by an objective lens having a numerical aperture not lower than 0.78, for example, 0.85.
When recorded information is reproduced from this optical disc, a light-receiving element receives returned light reflected on the optical recording layer 102 and a signal processing circuit generates a predetermined signal to obtain a reproduced signal.
The optical recording layer 102 of this optical disc has a concave and convex-like shape caused by the above-mentioned concave portion 101r formed on the surface of the disc substrate 101.
The concave portion 101r is formed as a continuous groove or a circular groove shaped like a spiral, for example, at a predetermined pitch, and the track area is divided by the concave and convex shape.
A concave portion and a convex portion of the concave and convex shape, which divides this track area, are generally referred to as a “land” and a “groove”. An optical disc can increase its storage capacity with application of a land and groove recording system for recording information both on the land and the groove. Moreover, only one of the land and the groove can be formed as a recording area.
The concave and convex shape caused by the concave portion 101r formed on the disc substrate 101 is formed as a pit having a length corresponding to recorded data so that the optical disc can be formed as a read-only memory (ROM) type optical disc.
It has been reported that tin oxide (SnOz, z<2) of metal oxide having a nonstoichiometric composition can be used as a recording material comprising an optical recording layer (for example see Journal of Materials Science Letters 19, 2000, 1833 to 1835).
It may be considered that this technology uses a phenomenon in which an optical constant of a light irradiated portion of an optical recording layer changes due to oxidation reaction caused when the optical recording layer is irradiated with light such as laser light.
However, when tin (Sn) is used as the recording material, if information is recorded on the optical disc by using an objective lens having a numerical aperture of about 0.8 to focus short-wavelength laser light having a wavelength ranging from about 380 nm to 420 nm on the optical disc, then a problem arises, in which recording marks of satisfactory shape cannot be formed so that jitter increases.